Density monitor and method

ABSTRACT

Methods and apparatus for determining density of gases and vapors. The primary sensing device is a fluidic oscillator through which a sample of gas is passed.

BACKGROUND OF THE INVENTION

This invention relates to determination of density of substances ingaseous form.

It is important to know the density of a gas in many industries, inparticular, in the area of petroleum and petrochemical processing. Atypical application is a mass flow meter, where volumetric flow rate iscombined with the density of the flowing stream to produce mass flowrate. One seeking to measure density, particularly on a continuouson-line basis, has a limited choice of apparatus. One commerciallyavailable density meter utilizes an oscillating element in the fluidwhose density is measured. Oscillation is caused by an electromagneticfield. The frequency of oscillation depends on the density of the fluid.The sensing element is contained in a housing having one-inch flangesfor installation in a pipeline. A standard reference, ProcessInstruments and Controls Handbook, 2nd ed., 1974, edited by Considine,lists only three techniques for measuring density, none of which arewell suited for use outside the laboratory. The listed methods (p.6-152) are as follows.

In a gas specific gravity balance, a tall column of gas is measured by afloating bottom fitted to the gas containment vessel. A mechanicallinkage displays movement of the bottom on a scale. A buoyancy gasbalance consists of a vessel containing a displacer mounted on a balancebeam and with a manometer connected to it. Displacer balance isestablished with the vessel filled with air and then filled with gas,the pressure required to do so being noted from the manometer in bothcases. The pressure ratio is the density of the gas relative to air. Ina viscous drag density instrument, an air stream and a stream of the gasunder test are passed through separate identical chambers, eachcontaining a rotating impeller. The two streams are acted upon by therotating impellers and in turn each acts upon a non-rotating impellermounted in the opposite end of the chamber. The non-rotating impellersare coupled together by a linkage and measure the relative drag shown bythe tendency of the impellers to rotate, which is a function of relativedensity.

STATEMENT OF ART

LeRoy and Gorland have explored the use of a fluidic oscillator as amolecular weight sensor of gases and reported their work in an articleentitled "Molecular Weight Sensor" published in Instruments and ControlSystems of Jan. 1971, and in National Aeronautics and SpaceAdministration Technical Memorandum TMX-52780 (circa 1970) and TMX-1939(Jan. 1970). In Fossil Energy I & C Briefs, Nov. 1981, prepared for theU.S. Dept. of Energy by Jet Propulsion Laboratory of CaliforniaInstitute of Technology, Sutton of The Garrett Corp., referred to theuse of a fluidic oscillator to measure mass flow. The use of a fluidicoscillator in measuring composition in a methanol-water system isdiscussed in an article on page 407 of Ind. Eng. Chem. Fundam., Vol. 11,No. 3, 1972. U.S. Pat. No. 3,273,377 (Testerman) shows the use of twofluidic oscillators in analyzing fluid streams. A fluidic device formeasuring the ratio by volume of two known gases is disclosed in U.S.Pat. No. 3,554,004 (Rauch et al.). In U.S. Pat. No. 4,150,561, Zupanickclaims a method of determining the constituent gas proportions of a gasmixture which utilizes a fluidic oscillator.

In National Aeronautics and Space Administration Technical MemorandumTMX-1269 (Aug. 1966), Prokopius reports on the use of a fluidicoscillator in a humidity sensor developed for studying a hydrogen-oxygenfuel cell system. In NASA TMX-3068 (June 1974), Riddlebaugh describesinvestigations into the use of a fluidic oscillator in measuringfuel-air ratios in hydrocarbon combustion processes. NASA Report No.L0341 (Apr. 16, 1976), written by Roe and Wright of McDonnell Douglasunder Contract No. NAS 10-8764 at the Kennedy Space Center, reports onwork done to develop a fluidic oscillator as a detector for hydrogenleaks from liquid hydrogen transfer systems. U.S. Pat. No. 3,756,068(Villarroel et al.) deals with a device using two fluidic oscillators todetermine the percent concentration of a particular gas relative to acarrier gas.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide methods and apparatus fordetermining densities of gases and vapors which are capable of use bothin the laboratory and in the field. Also, it is an object that suchapparatus be relatively inexpensive, have a minimum of moving mechanicalparts, and be compact, so as to facilitate transportation andinstallation. It is a further object of this invention that such methodsand apparatus have high reliability and accuracy while providing resultsessentially instantaneously. In one of its broad embodiments, theinvention comprises (a) a fluidic oscillator; (b) means for establishingflow of a sample through said oscillator; (c) means for measuring andcontrolling the pressure at which the sample passes through saidoscillator and transmitting a signal representative of the pressure; (d)means for measuring the temperature of the sample at said oscillator andtransmitting a signal representative of the temperature; (e) means formeasuring the frequency of oscillation at said oscillator andtransmitting a signal representative of the frequency; (f) computingmeans for calculating the density of the sample using equations and datastored in said computing means and data supplied by said means forproviding pressure, temperature, and frequency signals; and, (g) meansfor communicating information contained in said computing means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fluidic oscillator.

FIG. 2 is a schematic diagram of an embodiment of the inventioncomprising a density monitor wherein the density of gas flowing in apipeline is measured on a continuous basis and displayed in a remotelocation.

FIG. 3 is an expansion, in block diagram form, of the portions of FIG. 2labelled electronics.

DETAILED DESCRIPTION OF THE INVENTION

A device known as a fluidic oscillator is used in this invention. Thisis one of a class of devices which are utilized in the field offluidics. A fluidic oscillator may have any of a number of differentconfigurations in addition to that depicted in FIG. 1. The publicationsmentioned under the heading "Statement of Art" describe fluidicoscillators and their governing principles in detail and therefore it isunnecessary to present herein more than the following simpledescription.

A fluidic oscillator may be described as a set of passageways, in asolid block of material, which are configured in a particular manner. Ifthe passageways are centered in the block and the block is cut in halfin the appropriate place, a view of the cut surface would appear as theschematic diagram of FIG. 1. Referring to FIG. 1, a gas stream entersthe inlet, flows through nozzle 109, and "attaches" itself to one of twostream attachment walls 105 and 106 in accordance with the principleknown as the Coanda effect. Gas flows through either exit passage 107 orexit passage 108, depending on whether the stream is attached to wall105 or wall 106. Exit passages 107 and 108 can be considered asextending to the outside of the block of material in a directionperpendicular to the plane in which the other passages lie. Consider agas stream which attaches to wall 105 and flows through exit passage107. A pressure pulse is produced that passes through delay line 104.The pressure pulse impinges on the gas stream at the outlet of nozzle109, forcing it to "attach" to wall 106 and flow through exit passage108. A pulse passing through delay line 103 then causes the stream toswitch back to wall 105. It is in this manner that an oscillation isestablished. The frequency of the oscillation is a function of thepressure propagation time through the delay line and time lag involvedin the stream switching from one attachment wall to the other. For adelay line of given length, the pressure propagation time is a functionof the characteristics of the gas, as shown in the above mentionedpublications and also by the equations which are presented herein. Thefrequency of oscillation can be sensed by a pressure sensor ormicrophone located in one of the passages, such as shown by sensing port102. A differential sensing device connected to both passages can alsobe used. Sensing port 101 is shown to indicate one potential locationfor a temperature sensor.

The invention can be most easily described by initial reference to FIGS.2 and 3, which represent a particular embodiment of the invention.Reference will also be made to a particular prototype monitor which wasfabricated and tested. Referring to FIG. 2, gas is flowing throughpipeline 50. A sample flow loop 51 is formed by means of conduit, suchas 3/4-inch diameter pipe, connected to pipeline 50 upstream anddownstream of pressure drop element 53. The purpose of pressure dropelement 53 is to cause a loss of pressure in pipeline 50 which is thesame as the pressure drop in flow loop 51 when a sufficient amount ofgas is passing through flow loop 51. Gas flow through flow loop 51 issufficient when gas composition at sample point 54 is substantially thesame as that in pipeline 50 at any given instant. Normally pressure dropelement 53 is a device present in the pipeline for a primary purposeunrelated to taking a sample, for example, a control valve. A sufficientlength of pipeline 50 can serve as pressure drop element 53 or anorifice plate can be installed in pipeline 50 to serve the purpose.Valves 52 are used to isolate flow loop 51 from pipeline 50.

Pressure and temperature of the gas flowing in pipeline 50 are providedby pressure transmitter 75 and temperature transmitter 76. These arelocated close to pipeline 50, so that differences in pressure andtemperature between their locations and pipeline 50 are not significant.Pipeline 50 is covered with thermal insulation of a type commonly usedon pipelines. The location shown in FIG. 2 has the advantage of allowingthe density monitor to be a self-contained package. However, if thepressure and temperature differences are significant, transmitters 75and 76 can be located directly on pipeline 50. The measured pressure andtemperature are referred to herein as T₁ and P₁.

Sample line 55 carries a sample of gas from sample point 54 to fluidicoscillator 56. Filter 57 is provided to remove particles which might bepresent in the sample, so that the narrow passages of fluidic oscillator56 or other flow paths will not become plugged. Pressure regulator 58,of the self-contained type with an integral gauge, is provided so thatthe gas flowing through oscillator 56 is at a substantially constantpressure. The frequency of oscillation at oscillator 56 may vary withpressure, depending on the particular oscillator used and the actualpressure at the oscillator. As will be seen, frequency is correlatedwith density, so variation for any other reason is unacceptable. Anypressure regulating means capable of maintaining flow through oscillator56 at a substantially constant value may be used. Under certaincircumstances, sufficient pressure regulation will exist by virtue ofsystem configuration and pressure level, so that no separate pressureregulation device is needed.

Orifice 60 is provided for the purpose, in conjunction with pressureregulator 58, of maintaining a constant flow of gas through oscillator56. Pressure gauge 59 indicates the pressure downstream of orifice 60.Normally it is not necessary to install orifice 60, as sample line 55 orthe inlet port of oscillator 56 serves the same purpose. Conduit 71carries the sample away from oscillator 56, to the atmosphere in alocation where discharge of the gas will cause no harm or to a processvessel where it can be utilized. However, the quantity of gas issufficiently small that it may not be economical to do more thandischarge it to the atmosphere. Pressure transmitter 61 provides thepressure of the gas at the oscillator. Also, a switch device (not shown)which provides a signal for actuation of an alarm if the pressure doesnot remain in a previously established range may be provided. Thisswitch device would initiate communication that inaccurate results maybe obtained.

Obtaining a representative sample stream from a pipeline, providing itto the inlet port of a fluidic oscillator, removing it from the outletport of the oscillator, and maintaining a substantially constantpressure drop across the oscillator can be accomplished by a variety ofdifferent means and methods for each given set of conditions, such asdesired flow rate through the oscillator and pipeline pressure. Thesemeans and methods, which can be applied as alternatives to those shownin FIG. 2, are well known to those skilled in the art.

A fluidic oscillator can be designed and fabricated upon reference tothe literature, such as that mentioned under the heading "Statement ofArt" or may be purchased. In the prototype monitor, an oscillatorsupplied by Garrett Pneumatic Systems Division of Phoenix, Ariz. wasused. This oscillator is of a different configuration than that shown inFIG. 1 in that the "loops" formed by delay lines 103 and 104 are opensuch that the "loops" define cavities and in that there is only one exitpassage. Drawings of this configuration can be found in the citedreferences. The flow rate through this oscillator when testing naturalgas is approximately 250 cm³ /min when upstream pressure isapproximately 20 psig and the oscillator is vented directly toatmosphere. A flow rate range of 200 to 500 cm³ /min is considered to bereasonable for commercial use and sufficient to provide acceptabledensity results.

Temperature transmitter 67 provides the temperature of the gas at theoscillator. Any of the well known means of sensing temperature may beused for this and temperature transmitter 76, such as a thermister orthermocouple. A solid state semiconductor sensor was used in theprototype device. The sensor may be located in a passage of theoscillator, such as shown in FIG. 1 (sensing port 101), or in the sampleline or conduit adjacent to the oscillator. Microphone 66 senses thefrequency of oscillation at oscillator 56. It is located in a positionto sense when the gas stream attaches itself to one of the walls, suchas the position shown in FIG. 1 (sensing port 102). There are a widevariety of sensors which can be used, for example, a piezoceramictransducer, in which pressure induces a voltage change, or apiezo-resistance transducer, in which pressure induces a resistancechange. Used in the prototype was a Series EA 1934 microphone suppliedby Knowles Electronics of Franklin Park, Ill.

Signals from microphone 66 and transmitters 61, 67, 75, and 76 areprocessed by equipment denoted field electronics 68 and control roomelectronics 69. Field electronics are located adjacent to oscillator 56while control room electronics are in a central control room somedistance away from oscillator 56. This equipment processes the signalsto obtain densities of the gas samples and performs other functionswhich will be described herein. Display unit 70 receives signals fromcontrol room electronics 69 and communicates densities of the gas andother information in human-readable form. It may be, for example, aliquid crystal display. The information may be communicated to otherequipment, such as a strip chart recorder (not shown) for making apermanent record or a computer (not shown) for further manipulation.

Periodic calibration must be accomplished to check for malfunctions andchanges which might take place in the apparatus such as electronicdrift, corrosion, and substances accumulating in the apparatus. Twocontainers of calibration gas, 64 and 65, are provided to check that themonitor is operating properly. Normally one of the calibration gases hasa density in the lower part of the range of values expected of the gasflowing in pipeline 50 and one has a density in the higher part of thatrange. The monitor is placed in the appropriate calibration mode bymeans of one of input switches 18. By manipulating valves 63, 72 and 73,the calibration gases are allowed to flow, in turn, through calibrationconduit 62 and sample line 55 to oscillator 56. Since the pressure andtemperature of the calibration gases will vary as conditions such asambient temperature change, the calibration gas densities calculated bythe monitor must be adjusted to a pressure and temperature at which thecalibration gas densities are known. For example, if pressuretransmitter 61 measures a pressure of 20 psig and temperaturetransmitter 67 measures a temperature of 30° F. when calibration gasfrom container 64 is flowing and the density of container 64 gas isknown to be 0.0448 lb/ft³ at O° C. and 1 atmosphere, the densitycommunicated by the monitor must be at O° C. and 1.0 atmosphere. If thecommunicated density is significantly different from 0.0448, the monitoris not operating properly. Adjustment of a density value from onepressure and temperature to another is easily accomplished by means ofthe equation of state presented herein. The monitor may be arranged sothat densities of the calibration gases are displayed and a humantechnician must, if necessary, adjust the monitor to the knowncalibration gas densities, or may be arranged so that the monitor iscapable of adjusting itself. For example, as was done in the prototypedevice, the monitor could re-calculate the values of constants stored init which are used in calculating sample densities.

The procedure just described does not accomplish calibration of pressuretransmitter 75 and temperature transmitter 76. These items can becalibrated separately by standard means. If desired, the calibrationgases can be introduced into flow loop 51 upstream of these items inorder to include them in the calibration. It is also possible to comparea value determined by the monitor to the density of a calibration gas bymanual means. Pressure, temperature, and density could be communicatedby the monitor and an operator could refer to a standard chart or tablesto compare the communicated results to the actual density of thecalibration gas. Another method is to provide apparatus in line 55 toadjust pressure and temperature of calibration gas entering theoscillator to particular preestablished values. However, this methodwould be used only in rare circumstances, since it is less costly tomanipulate numbers than to manipulate the physical condition of thecalibration gases.

Partial calibrations, or operation checks, can be accomplished in anumber of different ways. Use of a calibration gas can be combined withoperation checks accomplished electronically. A totally electronicoperational check can be made. For example, means for generatingappropriate oscillating tones can be provided at microphone 66 so thatnew values of K₁ and K₂ can be calculated. Of course, this procedurechecks only the electronics and not the oscillator. In another simplecheck, a tuning fork is used to generate a tone at microphone 66 and thesynthetic "density" resulting from the tone input is compared to theexpected proper value in computing means. Temperature changes can beused to perform operational checks. This can be done by using heatingmeans, such as electrical resistance coils, to heat gas flowing into theoscillator and comparing densities of heated and unheated gas. If thegas used in the check is from a changing process source, provision mustbe made to prevent change during the checking period. This can beaccomplished by providing a container to collect a sufficient quantityof gas to do the check or recycling gas from the oscillator outlet backthrough the system. Given a particular objective to be accomplished,other checks will become apparent.

An assembly of electronics devices for processing signals from thetransmitters and microphone (variables) and providing signals to thedisplay unit can be fabricated from standard components by one skilledin the art. FIG. 3 shows one such design in simplified form. Line 19indicates which items are located in the field adjacent to oscillator 56and which are located in the central control room. A signal frommicrophone 66 is provided to amplifier 1, passed through filter 2, andconverted to a square wave pulse in square wave shaper 3. The output ofsquare wave shaper 3 is provided to counter 6 by means of transmitter 4and receiver 5. Counter 6 counts the number of cycles occurring inoscillator 56 in a unit of time, thus generating frequency information.The signals from pressure transmitters 61 and 75 and temperaturetransmitters 67 and 76 are selected one at a time by analog switchingdevice 7 and sent sequentially to analog-to-digital converter 8, wherethey are converted to digital form. Serial input/output device 9converts the output of analog-to-digital converter 8 to a serial pulsetrain, which is provided by means of transmitter 10 and receiver 11 toserial input/output device 12, located in the control room.

Memory device 15, a random access memory chip (RAM), is used to storethe variables. A program for control of the electronics devices andperforming computations is stored in memory device 14, a programmableread-only memory chip (PROM). Constants needed for the computation arestored in memory device 16, an electronically erasable programmableread-only memory chip (EEPROM). Central processing unit 13 performs thenecessary computations and provides output signals to display unit 70.Input switches 18 are used to provide human input to the electroniccomponents. These are rotary click-stop switches which can be set to anydigit from 0 to 9. One of the switches is the mode switch and the othersare used to enter numerical values. The position of the mode switch"instructs" the apparatus what to do. In the calculate mode, theapparatus displays the heat content of a sample. When the mode switch isplaced in the "constant load" position, numerical values of constantscan be manually set on the other switches and loaded into the system bydepressing a button. Another position of the mode switch allows valuesof variables to be displayed in sequence on display 70. When it isdesired to calibrate the apparatus, still other positions are used.Additional positions are used as required. Parallel input/output device17 provides a means of transmitting information from input switches 18and also controlling counter 6. It will be clear to one skilled in theart that certain of the electronics devices may be collectively referredto as a computer or computing means or may be contained within acomputer or computing means.

The basic equation used in the practice of this invention whichdescribes the operation of a fluidic oscillator is ##EQU1## M=molecularweight of the gas flowing through oscillator, G=specific heat ratio ofthe gas flowing through oscillator,

T=temperature of the gas flowing through oscillator,

F=frequency of oscillator output signal, and K₁ and K₂ =constants.

The quantity G can be provided as a constant stored in computer memoryor can be calculated by means of a correlation, such as the equation

    G=K.sub.3 +K.sub.4 M+K.sub.5 M.sup.2 +K.sub.6 M.sup.3,

where K₃, K₄, K₅ and K₆ are constants.

The density of the gas can be calculated by use of the equation ##EQU2##D=density, m=mass,

V=volume,

P₁ =pressure at the point of density measurement,

T₁ =temperature at the point of density measurement,

Z=compressibility factor, and

R=universal gas constant.

This equation is derived from the familiar equation of state ##EQU3##where n=number of moles. Z can be easily expressed by means of equationswhich depend on M and data available in the literature, as explainedherein.

The computer is programmed to solve these equations to obtain D, usingvalues of F, T, T₁, and P₁ provided as described above, and values ofconstants which exist in computer memory.

An approach to developing a basic oscillator equation on a theoreticalbasis is as follows. Reference is made to FIG. 1 as an example. Apressure pulse which passes through delay line 103 or 104, describedabove, travels at the local speed of sound, u. Denoting the length ofeach delay line as L, the time required for the pulse to traverse adelay line is L/u. The time for a complete cycle of oscillation includesthat required for a pulse to travel through each delay line. An equationfor the local speed of sound is ##EQU4## where u=speed of sound,

g=gravitational constant.

Thus the time required for the pulse to traverse the two delay lines is2 L/u or ##EQU5## As explained above, the total time for a cycle ofoscillation also depends on switching time, the time required forswitching of the stream from one attachment wall to another, or theperiod between arrival of a pulse propagated through a delay line atnozzle 109 and the start of a pulse through the other delay line.Switching time can be expressed as inversely proportional to u, that isas ##EQU6## Since L is a constant for any given oscillator and theinverse of time is frequency, the following equation can be written##EQU7## Solving the equation for M and making g, L, and R a part of theconstant, the equation becomes ##EQU8## If the above constant isdesignated as K₁, and K₂ is added to the right-hand side, the basicequation presented herein is obtained. It has been found necessary toadd the constant K₂ to the equation in order to accurately describe theoscillator. It is not possible to use a purely theoretical equation, inpart as a result of the imperfections of hardware and measuringequipment. For example, no two fluidic oscillators will perform in anidentical manner. In the prototype density monitor, which was developedfor use in a natural gas application, K₁ and K₂ were empiricallyestablished by flowing gases such as methane, ethane, propane, butane,and pentane through the monitor. The values of K₁ and K₂ thusestablished were 7.538×10⁶ and 1.58, respectively. This calibrationprocedure must be followed for each monitor which is fabricated, usinggases similar to the gas for which the monitor is to be used. However,only two calibration gases are required to define K₁ and K₂.

The equation for G used in the prototype unit was developed by astandard curve-fitting method using values of G available in theliterature for gases such as methane, ethane, etc. As can be appreciatedby those skilled in the art, there are other ways to develop and expressG and to store it in the computer. The most appropriate method isdependent on the particular application.

The compressibility factor, Z, is a measure of the deviation of thesample gas from ideality and is added to the expression commonly knownas the ideal gas law in order to make the ideal gas law applicable toreal gases. Since compressibility factors are covered by a vast quantityof literature which includes a number of different methods of computingthem, there is no need to explain the basic theory herein. For furtherinformation and references to the literature, refer to Basic Principlesand Calculations in Chemical Engineering, 2nd edition, 1967,Prentice-Hall, Inc., by Himmelblau, p. 149 and following. Also usefulare Chemical Process Principles, 2nd edition, 1954, John Wiley & Sons,by Hougen et al, p. 87, and Perry's Chemical Engineers' Handbook, 4thedition, McGraw-Hill, p. 4-49.

In the prototype device, Z is calculated by means of the equation##EQU9##

    Z.sub.B =0.999287+9.25222×10.sup.-5 M×1.06605×10.sup.-5 M.sup.2,

where

Z_(B) =Z at particular base conditions,

S=supercompressibility factor,

P₁ =psig, and

T₁ =^(o) R.

The equations for S are empirically derived. These and the equation forZ can be found in Principles and Practices of Flow Meter Engineering,9th edition, 1967, by Spink, published by Foxboro Co. and Plimpton Pressof Norwood, Mass. The expression for Z_(B) was derived by means ofcorrelating values of Z_(B) for gases of different molecular weights.This was done by converting values of base temperatures and pressuresfor various gases, using critical temperatures and pressures obtainedfrom the literature, to reduced pressure and temperature and then usingcharts prepared by Nelson and Obert to obtain Z_(B).

In a relatively simple embodiment of the invention, the sample loopshown in FIG. 2 is omitted. Sample is collected in an evacuatedpressure-resistant container, which is then connected to sample line 55,either upstream or downstream of filter 57. The density communicated bythe apparatus is that at the temperature and the pressure measured bypressure transmitter 61 and temperature transmitter 67. There is no needto divide the electronics into two packages at two different locations.This embodiment might be used in a laboratory. It might be desired toadd to this embodiment the feature that the apparatus is capable ofcalculating a density value for sample gas at pressures and temperaturesdifferent from those measured by transmitters 61 and 67 and which areprovided to the apparatus as follows. A temperature and a pressure canbe manually entered into the apparatus by means such as input switches18 or they can be provided by apparatus which measures temperature andpressure at some point of interest and transmits appropriate signals tothe computing means of the invention.

FIG. 2 shows a more complex embodiment of the invention where acontinuous flow of sample through the oscillator (at temperature T) isestablished in order to obtain a continuous density value for gasflowing in a process pipeline (at temperature T₁ and pressure P₁). Inthis embodiment, the apparatus is arranged to provide a densityrepresentative of the sample gas at a point upstream of the pressurecontrolling means represented by item 58 of FIG. 2 and further arrangedso that the upstream point is representative of the main stream fromwhich the sample is taken.

As noted earlier, a variation in the pressure at which gas passesthrough the oscillator may affect the accuracy of the monitor. This istrue even though the pressure is a variable used in calculating density;that is, a calculated density value may be incorrect if the pressurevalue used in the calculation is correct but outside a particular range.Therefore, it is desirable to monitor the pressure and communicate anydeparture from a previously established range. This can be accomplishedby several means, including adding a primary sensor, such as a pressureswitch, in the appropriate location, such as line 55 of FIG. 2, oradding the appropriate means in the electronics portion of the apparatusto utilize the pressure signal provided for use in the equation, such asthe signal transmitted by pressure transmitter 61 of FIG. 2. Thismonitoring provision is not depicted in FIG. 2.

The present invention may be embodied in apparatus for determining themass flow rate of gas in a pipeline. This can be done by combiningapparatus such as that shown in FIG. 2 with apparatus for measuring thevolumetric flow rate of the gas in the pipeline and multiplying densitytimes volumetric flow rate in apparatus such as the computing means ofFIG. 2. If the apparatus for measuring volumetric flow rate comprises acalibrated obstruction to flow, such as an orifice plate, and means tomeasure the pressure drop across the obstruction, such as a differentialpressure cell, the pressure drop can be provided to the computing meansfor calculation of mass flow rate instead of calculating the volumetricrate outside the computing means.

The term "gas" is frequently used herein; it should be understood toinclude vapors. The use of the examples set forth herein are notintended as a limitation on the broad scope of the invention as setforth in the claims. It is also intended that further applications ofthe principles of the invention as would normally occur to one skilledin the art to which the invention relates be included within the claims.

We claim as our invention:
 1. An apparatus for determining the densityof a gas comprising:(a) a fluidic oscillator; (b) means for establishingflow of a sample of the gas through said oscillator; (c) means forcontrolling the pressure at which the sample passes through saidoscillator; (d) means for measuring the temperature of the sample atsaid oscillator and transmitting a signal representative of thetemperature, said temperature being designated as T; (e) means formeasuring the frequency of oscillation at said oscillator andtransmitting a signal representative of the frequency, said frequencybeing designated as F; (f) means for determining the temperature of saidgas at the point at which said density is determined and transmitting asignal representative of the temperature, said temperature beingdesignated as T₁ ; (g) means for measuring the pressure of said gas atsaid point of density determination and transmitting a signalrepresentative of said pressure, said pressure being designated as P₁ ;(h) computing means for calculating the density of said gas by therelationship of P₁, T, T₁ and F in accordance with the equation ##STR1##wherein, D=density of said gasK₁ =a constant G=specific heat ratio ofsaid gas flowing through said oscillator T=temperature of said gasflowing through said oscillator P₁ =pressure of said gas at said pointof density determination F=frequency of oscillator output signalZ=compressibility factor R=universal gas constant T₁ =temperature ofsaid gas at said point of density determination; and (i) means forcommunicating information contained in said computing means.
 2. Theapparatus of claim 1 further comprising means for establishing acontinuous flow of sample through said oscillator.
 3. The apparatus ofclaim 1 further comprising a flow loop which is comprised of an inletconnection and an outlet connection communicating by means for a firstconduit wherein the inlet and outlet connections are connected to aprocess pipeline so that process fluid flows continuously through saidflow loop.
 4. The apparatus of claim 1 further comprising means formonitoring the pressure of the sample flowing through said oscillatorand communicating any departure from a previously established pressurerange.
 5. The apparatus of claim 1 further comprising means forestablishing a flow of one or more calibration gases, in sequence,through said oscillator and means for adjusting said apparatusresponsive to the known densities of said calibration gases.
 6. A methodfor determining the density of a gas comprising:(a) passing a sample ofsaid gas through a fluidic oscillator at a controlled pressure; (b)measuring the temperature of the sample at said oscillator andtransmitting a signal representative of the temperature, saidtemperature being designated as T; (c) measuring the frequency ofoscillation at said oscillator and transmitting a signal representativeof the frequency, said frequency being designated as F; (d) determiningthe temperature of said gas at the point at which said density isdetermined and transmitting a signal representative of the temperature,said temperature being designated as T₁ ; (e) measuring the pressure ofsaid gas at said point of density determination and transmitting asignal representative of said pressure, said pressure being designatedas P₁ ; (f) calculating the density of said gas by the relationship ofP₁, T, T₁ and F in accordance with the equation ##EQU10## wherein,D=density of said gasK₁ =a constant G=specific heat ratio of said gasflowing through said oscillator T=temperature of said gas flowingthrough said oscillator P₁ =pressure of said gas at said point ofdensity determination F=frequency of oscillator output signalZ=compressibility factor R=universal gas constant T₁ =temperature ofsaid gas at said point of density determination; and (g) communicatinginformation contained in said computing means.
 7. The method of claim 6further characterized in that numerical value of said constants K₁ andK₂ are determined by initial calibration with at least two initialcalibration gases having predetermined molecular weights.
 8. The methodof claim 6 further characterized in that said method comprisesintermittent calibration with at least one gas of predeterminedmolecular weight to periodically monitor the accuracy of the densitydetermination of said method.